Annealing effects on cathodoluminescence of zircon

Abstract

Color cathodoluminescence (CL) images of zircon from southern Malawi (MZ) show mottled yellow CL emissions on a dull luminescent background by the annealing below 600 °C, and relatively homogeneous blue emissions above 800 °C. CL spectroscopy of the samples reveals a change of the luminescent features with an annealing temperature, and explains a variation of their colors and luminance observed by a CL microscope. Emission bands at 310 and 380 nm in an ultraviolet (UV) to blue CL are assigned to intrinsic centers in host lattice, narrow peaks at 475 and 580 nm to Dy3+ impurity centers, and broad bands at 500 to 650 nm to Frenkel-type defects and SiOmn-groups. The former two show increases in emission intensities against annealing temperature, but the latter exhibits an increase in intensity up to 300 °C and a decrease above 300 to 700 °C. An increase in an annealing temperature leads to a reproduction of the intrinsic centers, which is responsible for an increase in UV-blue emission, in host lattice accompanied with a recrystallization from metamict state. An increase in the intensities of narrow peaks activated by Dy3+ impurities may result possibly from a recovery of ionization due to the self-radiation and an energy transfer of other REEs to Dy3+ activator. A gradual increase in yellow emission bands up to 300 °C might be caused by a migration of the hole around a thermally-instable lattice defect and/or activated impurities into a more stable site related to Frenkel-type defects and SiOmn-groups, whereas the yellow should be subsequently reduced due to an elimination of these defects.